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422 METALLURGY OF CAST IRON.
in thickness, it would be il white " and hard as flint. In the former case, also, it would show much less contraction than in the latter. The facts go to show that the length of time occupied in cooling a casting or that molten metal has solidified, may often be^inore effective in causing different degrees of contraction and hardness of -iron in a casting from ordinary used foundry iron, than any varying percentages of sulphur,'Silicon, etc., which exist in ordinary foundry iron. Any one giving due consideration to the points here raised will be led to concede the impracticability of formulating set rules for the contraction of castings, to be published as a universal guide to desired results in the dimensions of castings; but by a study of the phenomena here referred to, we will be in a fair position to determine what allowance should be made for contraction, etc., when we are on the ground of action. It is to be understood that reference is not made to the difference which may exist in the size of like castings from soft and hard iron, or variations due to the hardness of ramming and head pressure of molten metal on moulds, etc. We are mainly dealing with the elements involved in the question of contraction, as affected by rapidity o"f cooling, stretching of iron, and variations in the thickness of metal, etc., in castings.
Stretching is possible and due to influences exerted by conditions in casting, cooling, and forms of patterns, which overcome or retard free contraction. It can make castings larger than the patterns from which they were made, and it also makes it possible to obtain acceptable castings which could not be secured were it not for the fact that iron can be stretched. J I